THE TRUE NATURE OF IRAS-SELECTED, X-RAY-LUMINOUS NORMAL GALAXIES IN THE ROSAT ALL-SKY SURVEY

Luminous star-forming galaxies have often been suggested as potentiall y significant contributors to the cosmic X-ray background (XRB). Inter est in this possibility has been rekindled by a recently published sam ple of 244 IRAS/ROSAT galaxies that includes 20 with extreme X-ray lum inosities (L(X) = 10(42-44) ergs s-1) that are claimed to be ''normal' ' spiral galaxies. To investigate whether or not these 20 X-ray-lumino us spirals are truly normal star-forming galaxies, we have reexamined their classifications by obtaining new optical spectra of 13 of them, and by locating spectra in the literature for four. Our results indica te that 13 of the 17 objects are previously unrecognized Seyfert galax ies. Of the four star-forming non-Seyfert galaxies found in this sampl e, three are incorrectly identified as X-ray sources. Only one H II ga laxy is a confirmed X-ray source, but it has L(X) almost-equal-to 10(4 2) ergs s-1 and is only about twice as luminous as the most luminous n ormal spirals detected previously at X-ray wavelengths. Thus, there ar e no H II galaxies with L(X) substantially in excess of 10(42) ergs s- 1, and claims of a new class of X-ray-luminous spiral galaxies are not supported by this study.